So-called unsupervised operation, among other modes, is possible with office machines of the type mentioned, which normally have an electronic control unit consisting of at least one microprocessor, a ROM memory containing the control program and a RAM memory receiving the variable data. For this reason it is necessary to generate a ribbon end signal before the start of a printing command or typing on the typewriter, in case the ribbon comes to its end, and to lock the printing mechanism and/or the keys.
In connection with the ribbon cassettes used in these machines it is already known from U.S. Pat. Nos. 4,494,886 and 4,655,624 to place a detector unit opposite a recess in the ribbon cassette housing for the purpose of detecting the end of the ribbon, through which the ribbon can be scanned, and thus a ribbon end signal is generated by the detector unit by means of a reflecting layer on the ribbon.
It is further known from U.S. Pat. No. 4,491,430 to provide a propeller driven by the ribbon for monitoring the ribbon, the vanes of which interrupt the signal path of a photoelectric barrier either directly or via drive members. When the signal change at the photo-electric barrier stops, for example when the end of the ribbon has been reached, the control unit monitoring the signal change generates a corresponding signal. Additionally, the core of the ribbon, which is movably supported in the ribbon cassette, blocks the propeller when reaching a predetermined amount of ribbon remaining and this can be used as described above.
Besides the absolutely necessary requirement of determining the end of the ribbon, it is also desirable in view of completing long print orders, perhaps in the unsupervised mode, to perform as accurate an estimate as possible of the amount of ribbon still available prior to the start of the print order. Up to now this was done, for example, by direct visual inspection, which required the opening of the housing. This in itself shows that this method is very cumbersome. However, the greatest disadvantage is the inherent inexactness of this technique.
To overcome these disadvantages, it is known from copending application Ser. No. 002,745 (owned by the present assignee) to estimate the amount of ribbon remaining by counting the ribbon advance steps made during the turning of the ribbon spool through a preset turning angle. For this purpose it is necessary to provide an incremental angle encoder on the ribbon cassette which can be read by means of a sensing device provided on the machine. The sensing signals thus produced are evaluated by an electronic control unit.
It is further known from U.S. copending patent application Ser. No. 061,311 (owned by the present assignee) to provide in the ribbon cassette a ribbon spool sensing element which is under the influence of a spring and which, for example, acts on a potentiometer so that the resistance that can be picked up constitutes a measurement of the amount of ribbon remaining.